Metastatic high-grade pulmonary artery sarcoma, with the foremost specific considerations being leiomyosarcoma and intimal sarcoma. The original tumour in the right lower lobe of the lung involved and occluded the pulmonary artery, leading to the initial presentation of shortness of breath and hemoptysis. A very small biopsy at the time was suspicious for a sarcoma with osteosarcoma-like elements, but a skeletal survey subsequently demonstrated no bony lesions. The metastasis features extremely pleomorphic histology, with a morphology similar to some high-grade gliomas but with a very well-demarcated interface with adjacent cerebral parenchyma and a subtly chondroid matrix.
The imaging impression of pulmonary arterial involvement and invasion is highly suspicious for a pulmonary artery intimal sarcoma (PAIS), while immunoexpression of smooth muscle actin (SMA) and desmin and lack of MDM2 overexpression (seen in at least 70% of PAIS) also prompts consideration of a pulmonary artery leiomyosarcoma. A rarer possibility, given the lack of detectable primary osseous lesions, would be that of a primary pulmonary extraskeletal osteosarcoma.

Additional relevant investigations and comment.
IHC: Ruling out primary high-grade gliomas, metastatic carcinomas, and melanomas is essential, with GFAP, Olig2, cytokeratins, and S100/ SOX10/MelanA/HMB45 immunohistochemistry representing a helpful panel. Though variable, PAIS cases tend to express vimentin and SMA, while desmin can be variable. MDM2 is overexpressed in approximately 70% of PAIS cases, though this particular case instead demonstrated immunohistochemical evidence of ATRX and TP53 mutations.
Leiomyosarcomas strongly express vimentin, SMA, and desmin but not myogenin (which would instead be suggestive of a rhabdomyosarcoma), and commonly carry TP53 and ATRX mutations. Both mesenchymal neoplasms originate from the wall of the pulmonary vasculature and have similarly poor prognoses.
Comment: Brain metastases from sarcomas are extremely rare, and epithelioid morphology in no way precludes the diagnosis. Due to possible overlapping morphological and immunohistochemical features, the delineation of metastatic sarcomas requires careful correlation with imaging.

References
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Li et al. (2021). Pulmonary artery intimal sarcoma mimicking pulmonary thromboembolism. Medicine (Baltimore); 100(6):e24699. doi:10.1097/MD.0000000000024699.
Kato et al. (2021). Pulmonary artery intimal sarcoma diagnosed preoperatively by endovascular biopsy and treated via right pneumonectomy and pulmonary arterioplasty. Case Rep. Pulmonpol.; 5573869. doi: 10.1155/2021/5573869.
Chen et al. (2016). Clinicopathological and immunohistochemical features of pulmonary artery sarcoma: a report of three cases and review of the literature. Oncol. Lett.; 11(4):2820-2826. doi:10.3892/ol.2016.4308.